Active matrix liquid crystal displays for computers and the like provide significantly improved performance; however, the cost of such displays is also a significant factor in the overall price of portable computers using the same. In addition, active matrix displays require substantially more power than passive displays since this type of display utilizes backlighting.
An active matrix liquid crystal display is constructed from two components that are separately constructed and then bonded together. The first component consists of an array of thin film transistors (TFTs) fabricated on a glass substrate, one per pixel. Each TFT has an associated capacitor which stores a potential value which is then applied to a liquid crystal element which passes a fraction of the light incident thereon, depending on the applied voltage. The TFTs are connected by transparent row and column electrodes, which are used to select each pixel. The liquid crystal layer together with color filters are fabricated on a second glass substrate. An adhesive is screen-printed around the edge of one glass plate, and an optical aligner is then used to bring the two together with the TFTs aligned to the filters. The adhesive is then UV (ultraviolet) cured and the liquid crystal material introduced by capillary action. The polarizers and a backlight are attached to the outside of the display.
Displays based on organic light emitting devices (OLEDs) have the potential for substantially reducing the cost of computer displays. OLEDs are emissive displays that provide an alternative to other types of light emission, such as vacuum fluorescence, plasma, inorganic electroluminescense, and inorganic light emitting diodes. Since a pixel only draws power when “on”, a display based on OLEDs requires less power than a backlight-based display.
OLEDs are constructed on a transparent substrate coated with a transparent conducting material, such as Indium Tin oxide (ITO), one or more organic layers and a cathode made by evaporating or sputtering a metal of low work function characteristics, such as Ca or Mg. The organic layers are chosen so as to provide charge injection and transport from both electrodes to the electroluminescent organic layer (EL) where the charges recombine emitting light. Often there is at least one or two organic hole transport layers (HTL) between the ITO and the EL, as well as at least one or two electron injection and transporting layers (EL) between the cathode and the EL.
While OLEDs have the potential for reducing display costs, prior art methods for fabricating OLEDs have several disadvantages when applied to active matrix displays. In principle, a set of TFTs and conductors on a substrate can also be utilized for constructing an active matrix display based on OLEDs. After constructing the TFTs the pixels are defined by depositing a patterned conductor corresponding to either the cathode or anode of the OLED. The organic light emitting layers and associated transport layers are then deposited, followed by a uniform layer of the other OLED electrode (anode or cathode). Such structures are described in detail in Eugene Y. Ma, et al., Soc. Inform. Display Conf. Proc. Sep. 15–19, 1997 (Toronto, Canada), p.L78.
While such a process has the potential to produce useful displays which may be competitive with AMLCDs, it has several disadvantages. First, the processing of the organic LED must be done on the same substrate as used for the TFTs, typically glass. As a result, the large cost advantages associated with fabricating polymer LEDs on flexible plastic substrates with roll-to-roll equipment cannot be realized. Ma, et al., attempt to overcome this limitation by using stainless steel foil as a substrate. In this method, the TFTs and OLEDs are constructed as described in the previous paragraph, using stainless steel foil in place of glass. However, since this substrate is opaque, the top electrode of the OLED must be transparent. The preferred transparent electrode material is indium tin oxide (ITO) which is deposited by sputtering. Hence, the method suggested by Ma, et al. requires the active organic layers to be subjected to the hostile conditions associated with the sputtering of the ITO. This leads to damage of the active polymer layers. In addition, this architecture requires the organic material to be deposited on top of the cathode. However, the cathode must be constructed from a low work function material such as Ca or Mg which is easily oxidized.
Broadly, it is the object of the present invention to provide an improved OLED-based active matrix display.
It is a further object of the present invention to provide an OLED-based active matrix display that can be fabricated utilizing roll-to-roll processing techniques on polymer films.
It is a still further object of the present invention to provide an OLED-based active matrix display which does not require the deposition of the transparent electrode onto the already deposited organic layers.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.